The short-term objective of this investigation is to determine if the oral administration of acetylsalicylic acid (aspirin) will result in a reduction or cessation of transient ischemic attacks of the hemispheric or monocular type. Reduction in the occurence of cerebral infarction and/or myocardial infarction is a long-term goal. Aspirin induces a marked delay in the aggregative properties of platelets. This anti-aggregation action of aspirin has been correlated with a reduction in experimental arterial thrombosis and in venous thromboembolism in patients undergoing cup arthoplasty of the hip. A cooperative group working for 10 years in the field of transient cerebral ischemic attacks due to extracranial vascular lesions has found evidence that plateletfibrin embolisms from non-obstructing and obstructing lesions in the extracranial circulation are responsible for many ischemic events. Interest has therefore focused on prevention of arterial embolism and thrombosis by administration of drugs which are capable of inhibiting platelet aggregation. To date there have been no controlled clinical trials of aspirin, although several promising uncontrolled clinical studies have been undertaken in England and Canada. Development of this controlled clinical trial was prompted by these clinical experiences and laboratory evidences which suggest that platelet aggregation inhibitors may play an important role in preventing intraarterial thromboembolism.